Certain mobile phone systems use a memory unit for storing a voice message from a user such as an outgoing voice message. The user can store their personalized voice message in that memory. The message is played back as the user's personalized message. That memory unit has a limited size. However, the user often wants to store a longer message.
Speech is often compressed before storing in this memory. Speech can be compressed using a well-known voice coder or vocoder system. Vocoders are well known in the art and compresses the speech by using well-known models. These models typically compress the speech based on its different parts: silence, noise and actual utterances. Codebooks are used to compress the different components of the speech.
A well-known and often-used system compresses the speech using variable rate coding. Several different standards exist. The Enhanced Variable Rate Coding ("EVRC") from Telecommunications Industries Association (TIA) is one. Cellular carriers in Japan including DDI and IDO have decided to adopt EVRC. Qualcomm also has a system called the Qualcomm Code Excluded Linear Predictive Coder ("QCELP").
A basic description of such variable rate codecs is found in U.S. Pat. No. 5,495,555, the disclosure of which is incorporated by reference to the extent necessary for proper understanding.
All of these systems operate an algorithm that determines an optimal encoding rate to use depending on the voice power.
The rate can be encoded at one-eighth maximum rate for much of the signal. Other parts of the signal can be encoded at one-quarter, one-half, and/or full voice power as shown. Again, the algorithm determines this coding. By reducing the maximum voice power that is encoded, the system saves on bandwidth.